1. Field of the Invention
This invention relates a head positioning control method and a head positioning control device for positioning a head at a target position of a storage medium, and particularly to a head positioning control method and a head positioning control device for reducing the seek time.
2. Description of the Related Art
Disk devices that have a head for reading disk-type storage medium are widely used. For example the magnetic disk drives used as a storage devices for computers comprise a magnetic disk, a spindle motor for rotating the magnetic disk, a head for reading from and writing to the magnetic disk, and a VCM actuator for positioning the head on a track of the magnetic disk. The storage density of these kinds of disk drives is rapidly increasing, as well as is the track density of the magnetic disks. Therefore, it is necessary to perform high-precision positioning at high velocity.
When a read or write command is received from the computer, the disk drive moves the magnetic head from its current position to the target position. This is called the seek operation. This seek operation is transition operation which moves from coarse control to following control by way of settling control.
Coarse control controls the velocity to the target position. Coarse control comprises velocity control, PD control, or observer control that does not include steady-state bias estimation. There is no integral element (integral compensation or bias compensation) included in the control system. Coarse control switches the control mode among acceleration, constant velocity and deceleration. In the acceleration mode, current flows to increase the velocity. In the constant-velocity mode, the current is ‘0’ or a previously measured bias compensation current flows, to keep the velocity constant. In the deceleration mode, the current flows in the opposite direction of the current during acceleration, to bring the velocity to almost zero near the target position. When the distance is very short, the constant-velocity mode is eliminated.
Following control controls the magnetic head so that it follows the target position. Following control comprises PID control, PI×Lead Lag, or observer control that includes steady-state bias estimation. This control is characterized by there being an integral element (integral compensation or bias compensation) included in the control system. Settling control is the control mode that connects coarse control with following control. In settling control, there may be an integral element in the control system.
The average time required for this seek operation for a 2.5-inch HDD is on the order of 10 of milliseconds. As the time required for coarse control is determined according to the distance of movement, and the maximum current value of the power amp, it is difficult to reduce this time. In order to reduce the seek time, it is necessary to reduce the time required for settling control and short distance seek.
The following prior method has been proposed for shortening this settling time.
First, a steady-state bias (external force) applied to the actuator influences the settling time. In other words, the actuator is applied constantly a steady-state bias (external force) from an external. To output the head signal from the actuator, a flexible cable (FPC) is provided to the actuator. This FPC shows spring-like characteristics. Moreover, as the disk rotates, the wind that is generated is applied to the actuator. These external forces also affect the friction and grease condition of the actuator bearings. The bias of the actuator due to these causes changes depending on the position of the head. When this bias value is not corrected, the settling time becomes long.
Therefore, in the prior art, an average bias value is measured beforehand for each position and stored in a table, then the value from the bias table, Bias Table (y[k]), that corresponds to the detection position y(k) is read and the instruction current value u(k) is corrected. (Disclosed for example in Japanese Unexamined published patent No. H2-232875 and H11-25623)
In addition, since the bias value fluctuates, a method of estimating the shift in this bias value during settling time, and correcting the instruction current value is also known. This shift is corrected by an integrator or a bias estimator in the control loop. When using an observer control system, a method of setting the current position and velocity as the initial observer value at the start of settling, and setting the estimated bias value to ‘0’ is performed. The reason for initializing the estimated bias value to ‘0’ is because the shift in the bias value is not known at the beginning of settling, so it is set to ‘0’. In this method, the current output is the sum of the status feedback current and the pre-measured bias value from the table, so the estimated bias value gradually converges according to the status feedback response, and so the position also converges to the target position.
Second, it has been attempted to shorten the settling time by changing the response characteristics of the feedback during settling control. For example, there is the method of setting the gain of the integrator in the loop to ‘0’ or decreasing it during settling control (as disclosed in Japanese unexamined published patent No. H2-278582), or the method of increasing the gain in the loop during settling control (as disclosed in Japanese Unexamined published patent No. H7-153211 and H1-133272).
Moreover, it has been proposed to use feed-forward control in settling control and short-distance seeking, in which a feed-forward current is made to flow for correcting the position offset on the target trajectory, and thus making it possible to shorten the time required for settling control and short-distance seek. For example, this is proposed in Japanese unexamined published patent No. H9-139032 or in the technical report ‘Fast Seek Control Taming Actuator Vibrations for Magnetic Disk Drives’ (IEEE Intermag 99).
However, in the prior art, there are the following problems. FIG. 56 shows the characteristics of the steady-state bias.
(1) As shown in FIG. 56, the steady-state bias value not only changes depending on the position of the head, but that bias value also differs, even for the same track, depending on the where the head was located in the past. In other words, as shown in FIG. 56, it has hysteresis characteristics. Therefore, in the prior method of using a bias table of average bias values, the shift in bias values is large. For example, for a 2.5-inch HDD, there is 1 to 3 mA error. Since this error cannot be estimated in advance, correction is not possible. Therefore, in a feedback control system in settling control, there must be time to correct this shift in bias value, and so there is the problem of not being able to shorten the settling time.
(2) Moreover, in the prior method of using a bias estimator, this shift in the bias value is estimated and corrected. However, since the bias estimate at the beginning of settling is not known, the initial bias estimate value is set to ‘0’. Therefore, it takes time for the estimated bias value to converge to the correct value. Also, during this time to convergence, there is a shift in bias value, so as a result of estimating and correcting for shift, overshooting or undershooting occurs. Therefore, it takes time to correctly estimate the bias value, and so there was the problem of not being able to shorten the settling time.
(3) Furthermore, in the prior method of changing the response characteristics during settling control, the dynamic characteristics of the feedback control system are improved, however the position error increased and position offset occurred, making it impossible to shorten the settling time. Also, since it is necessary to correct the shift in the bias value, there is the problem of not being able to shorten the settling time.
(4) In the feed-forward control, the velocity at the start of feed-forward is not considered and is assumed, for example, to be zero. In addition, the prior idea is thought that the velocity error could be suppressed by feedback control. However, the track width becomes more narrow and it is not possible to ignore the effect on the velocity even when there is only small external vibration. When the velocity is large, there is the problem that it takes time for convergence, and it is difficult for the velocity error to be restored within the target time. The effect of this velocity error is especially large during short-distance seek, and in normal feedback control the convergence time is approximately 2 to 3 ms. For short-distance seek, it takes 1 ms or less to seek one track, for example, so there is the problem that the effect of velocity error is large, and that it is not possible to shorten the convergence time.